White cement clinker, like the usual Portland cement clinker, is formed by burning a mixture of solids in a rotary kiln or furnace to sinter the particles together. Unlike Portland cement, however, the cooling of white cement clinker requires special care to avoid any deterioration of the degree of whiteness of the product.
In German patent document (printed application) No. 1,942,537 for example, for a given composition, a cooling duration of 1.5 to 6 minutes is prescribed to bring the temperature to 700.degree. to 1000.degree. C. whereupon rapid cooling is carried out under a reducing atmosphere to about 300.degree. C. by spraying with water.
In the process described in German Pat. No. 1,178,769 a fuel is sprayed upon the clinker to function as a reducing agent and directly thereafter the product is quenched by spraying the clinker within the outlet of the rotary furnace with water.
German patent document (open application) 28 13 325 directs the predominant proportion of the fuel as a stream directly upon the growing clinker. Recent investigations (see Zement-Kalk-Gips 6/78, pages 291 through 293) have shown that it is important to carry out the first cooling phase rapidly to a temperature of about 1200.degree. C. under a reducing atmosphere to avoid coloration of the product. It has also been found that only with this cooling approach is it possible to prevent loss in strength of the cement when the latter is ultimately used, such loss of strength characterizing complete cooling of clinker in water.
Other tests have demonstrated that the superficial treatment of the clinker with water can only avoid coloration if the clinker is in a finely divided form, i.e. has been previously converted to small granules.
If granulation is not effective to produce a product of a sufficiently small particle size, internal portions remain unquenched and develop a brown color which adversely affects the milled product.
Even if granules of a diameter of 15 mm or more are cooled by superficial spraying, for example, it is found that core portions remain sufficiently hot and cool at such a low rate that a brownish color agent develops.
It is thus common in the cooling of white cement clinker to convey the latter from the sintering zone into a water bath and to allow the clinker to fully cool by immersion therein.
This, of course, has disadvantages with respect to the thermal economy of the process because the residual heat of the clinker is completely lost, thereby increasing the thermal demand of the process by 20 to 30%.
Another disadvantage is that the period between leaving the sintering zone to entering the bath may require several minutes over which the clinker can cool through several hundred degrees and a portion of the divalent iron is oxidized. This, of course, results in a loss of whiteness.
Still others (see German patent documents - open applications Nos. 11 78 325 and 28 13 325) have suggested the quenching of the clinker in the rotary kiln as soon as the clinker emerges from the sintering zone with water in sufficient quantities that the small granules pick up some moisture, i.e. are practically completely cooled, whereas the large granules and chunks remain hot at their cores. A cooling drum is then provided downstream of the rotary furnace in which temperature equalization can be effected so that the residual heat suffices to vaporize all of the moisture pick-up by the clinker. This residual heat, therefore, is not available to heat the combustion air drawn through the drum into the oven.
These techniques may sufficiently quench only the fine granules, generate water vapor which must be drawn through the furnace and detrimentally affects the heat balance thereof, and may not satisfactorily cool the larger pieces of clinker. Consequently, the larger portion of the granulate and the chunks must be removed so as to prevent any detrimental effect upon the degree of whiteness of the cement.
These systems also reduce the strength of the cement.